CAR TALK: Gas mileage drops in winter for lots of reasons; here’s what to do about that

DEAR CAR TALK: My mileage drops in the winter. I understand this has something to do with winter, but my question is: What is it about winter that makes mileage drop? — Mary

DEAR MARY: A bunch of things. Some mechanical, some behavioral.

When it's cold out, you may spend more time warming up your car. Even if you don't sit in the driveway and let it warm up, it takes the engine longer to get to full operating temperature, where it runs most efficiently. Then it cools down more quickly between trips.

In the winter, you may drive places to which you'd ordinarily walk. If you go to a store and need to stop at another store four blocks away, you'd probably enjoy the walk in the summertime. When it's 25 degrees out, you might drive. And short trips produce poorer mileage.

For every 10 degrees Fahrenheit the temperature drops, your tire pressure drops about 1 PSI. Even if it doesn't drop enough to turn on the tire pressure warning light, a drop of 2 or 3 PSI will lower your mileage a bit.

When the car is cold, your oil and other engine fluids are harder to move around, which creates more internal engine friction. That also reduces your mileage.

You may be using heated seats, front and rear defrosters, and other accessories that use power. When you use more power, your mileage takes a hit.

Your battery is less efficient in cold weather, forcing the alternator to work harder to keep it charged up.

And finally, in many parts of the U.S., winter blends of gasoline have slightly less energy per gallon than summer blends.

Add it all up, and mileage drops noticeably in cold weather.

What can you do?

1. Overinflate your tires a little bit. If they call for 32 PSI, set them at 35. No harm will be done, and when their pressures drop due to temperature, they'll be closer to where they should be.

2. Minimize warm up. Unless it's subfreezing, start the car and go. If it's 20 F out, let the car warm up for 30 seconds. But skip the unnecessary three-minute warm up, and especially the remote, 20-minute warmup while you wolf down two scrambled eggs and a stack of pancakes.

3. Relocate to Arizona. That'd be my choice.

DEAR CAR TALK: I have a 2012 Honda Accord. It was driving fine until recently when it started hesitating and then jerking (like a manual transmission that goes too slow in a high gear).

I took it into a reputable shop, and they replaced something to do with "the timing" (I'll admit I didn't pay attention). It solved the jerking issue, but there is still something weird going on.

When the car is cold or hasn't been running for a while, it starts just fine. However, if I've been driving for a bit, for example to the grocery store, when I come out and try to start the car again, it takes a couple of "errr errr errr" noises before it starts.

So, it starts easily when cold, but not when warm. What in heaven's name is going on? I was hoping to give the car to my college daughter, but now I'm afraid it'll die on her. — Regina

DEAR REGINA: Well, if it dies before she ever gets it, it'll prove to be a very safe car for a teenager.

I'm not sure your timing and starting problems are related. There's no adjustment on your timing chain. So, there's nothing the shop could have adjusted incorrectly there.

If we approach your hot start issue as a fresh problem, the first thing to check for is a weak battery. So have the charging system tested.

Next, there are several relays that could be sticky or failing. Sometimes, one weak solder joint in the relay is all it takes. There's the fuel pump relay. Without the fuel pump, no fuel gets to the cylinders. So, a delay in closing that relay could cause a slow start. We've seen this on a lot of Accords of your era. The other key relay is called the main engine relay, which allows power to flow to the computers, sensors, and other stuff that let the car start and run.

Each of those relays costs about $50. So if your battery and charging system check out, and your mechanic is still stumped, have him try replacing both of those relays and see if it solves the problem.

Ray Magliozzi dispenses advice about cars in Car Talk every Saturday. Email him by visiting cartalk.com

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